The Sun and the Moon are considered the two illuminating planets but with different nature- Source and Reflector. One possible explanation may be as under:
The Sun and the Moon represents Purush and Prakruti respectively and thus owns one sign each representating the above basic elements. The rest five planets, have two signs each of which one is Purush (or Male in simple terms) and the other is Prakruti (or Female in simple terms). Secondly, if we go ahead from the sign of the Sun or go backward from the sign of the Moon, we get the same ownership as per arrangement of that planet in Solar System. For eg. the first ownership ( Gemini and Virgo) is of Mercury the inner most planet and the last ownership (Capricorn and Aquarius) is of Saturn the outer most planet.
Thanks for sharing the info. In fact, in one of the book i read the other planet's order from sun/moon(as you mentioned).
All these days, i was thinking that since sun and moon are significator of father and mother respectively. A child can have only one father and mother, where as it can have multiple siblings/house/career.. etc. which other planers are significator of.
@Other member,
Please share your opinion/provide more information on this.